1. Field of the Invention
The present invention relates to oil recovery from fractured reservoirs, the major part being contained in porous matrix blocks limited by fractures.
2. Description of the Prior Art
In the case of weakly water-wet or non-water-wet fractured reservoirs, water injection is often ineffective because the capillary forces retain the oil within the matrix blocks.
Adding a chemical additive to the injection water can then be considered in order to modify the interfacial properties of the water-oil-rock system, according to two potential procedures which are capillary force intensity reduction through water-oil interfacial tension reduction and restoration of capillary motive forces through wettability inversion of the rock, which becomes water wet again in the present case.
Injection of additives increasing the viscosity of the water injected can also be considered in order to increase the pressure gradient within the fluid circulating in the fractures and thus to carry along the oil in the matrix blocks through viscous effect.
In some cases, it is also possible to use another recovery method involving the injection of a gas alone or in combination with an aqueous phase and/or a chemical agent (alternate water/gas or foam injection for example).
However, selecting and implementing one among these many enhanced recovery methods first requires sufficient evaluation of the context of the reservoir being studied and of the effects of the production mechanisms being used.
This notably involves in estimating the impact of the physical mechanisms likely to allow oil expulsion from the matrix blocks. That is, in the case of production by injection of a fluid (most often water) maintaining the pressure, (a) the gravity forces related to the presumed height of the blocks, (b) the forced (viscous) displacement within the blocks associated with the circulation of the fluid injected and/or produced in the fractures, and also in some cases (c) exchanges by diffusion caused by possible composition gradients between fracture and matrix. The expansion mechanism associated with the compressibility of the fluids and of the rock can also be involved if the pressure and/or the temperature of the reservoir vary during production. The capillarity forces are not mentioned as a recovery-promoting mechanism because the reservoirs concerned here are preferably oil wet, which are the most frequently encountered ones in a carbonate context.
Evaluation of these various recovery mechanisms, notably the gravitational effects and the viscous effects likely to compensate for the opposing effects of an unfavorable capillarity in the case of water injection in an oil-wet fractured medium, is essential for enabling the reservoir engineer to choose an enhanced recovery method that amplifies or restores the beneficial impact of one or the other of these mechanisms. It can notably be a chemical process comprising adding a chemical additive to the injection fluid that is most often based on water.
This prior evaluation allows, on the one hand, to select an enhanced recovery strategy and, on the other hand, to focus the means and efforts on the sizing and optimization of the method(s) meeting the requirements relative to the enhanced recovery strategy that is selected.
There are known methods of evaluating oil recovery in fractured oil fields. These methods are based on statistical, analytical, experimental and/or numerical approaches. Such methods are for example described in the following documents:    Reis, J. C.: “Oil Recovery Mechanisms in Fractured Reservoirs During Steam Injection”, paper SPE 20204 presented at the SPE/DOE EOR Conference, 22-25 Apr. 1990, Tulsa, Okla.    Jack Allan and S. Qing Sun: Controls on Recovery Factor in Fractured Reservoirs: Lessons Learned from 100 Fractured Fields, paper SPE 84590 presented at the 2003 SPE Ann. Tech. Conf. & Exh. held in Denver, Co., USA, 5-8 Oct. 2003.    Barbara Pirker, Georg M. Mittermeir and Zoltan E. Heinemann: “Numerivally-Derived Type Curves for Assessing Matrix Recovery Factors”, paper SPE 107074 presented at the SPE Europec/EAGE Ann. Conf. & Exh. held in London, UK, 11-14 Jun. 2007.    R. Gupta, B. Adibhatla and K. K. Mohanty: Parametric Study to Enhance Oil Recovery Rate From Fractured Oil-Wet Carbonate Reservoirs, paper SPE 116485 presented at the 2008 SPE Ann. Tech. Conf. & Exh. held in Denver, Co., USA, 21-24 Sep. 2008.    B. Bourbiaux: “Understanding the Oil Recovery Challenge of Water Drive Fractured Reservoirs”, paper IPTC 13909 presented at the International Petroleum Technology Conference, 7-9 Dec. 2009, Doha, Qatar.    Nicolas Legrand, Joop de Kok, Pascale Neff, and Torsten Clemens, OMV E&P: “Recovery Mechanisms and Oil Recovery From a Tight, Fractured Basement Reservoir, Yemen”, SPE Reservoir Evaluation & Engineering, August 2011.
Patent applications WO-10,002,680 and WO-10,138,558 relate to methods of evaluating an enhanced oil recovery technique.
However, all these methods require a large amount of data, which is sometimes difficult to obtain, and/or they generally use reservoir simulations. These methods are therefore long and complex to implement.
Furthermore, for typical fractured reservoirs, the problem of recovery arises specifically on the matrix block scale because these matrix blocks contain almost all of the oil and they are bypassed by the fractures.